For example, with reference to JP-A-9-510416 (U.S. Pat. No. 5,710,633), a rain sensor (raindrop detection device) of a reflection type is provided. In this case, light is irradiated form a light irradiating portion to a windshield. Reflection light reflected at the windshield is detected by a light receiving portion. Rainfall is detected according to a phenomenon that the amount of the reflection light varies due to the amount of raindrop adhered to the surface of the windshield.
In this case, because of the decrease of the light reflection and the like at the inner surface of the windshield, it is desirable that air layer is not mediate at the light path along which light is back to the light receiving portion from the light irradiating portion through the outer surface of the windshield. Thus, the rain sensor in which the light irradiating portion and the light receiving portion are embedded is pressed against the inner surface of the windshield through a soft translucent sheet. That is, a construction making the rain sensor tightly contact the windshield is used.
Specifically, the rain sensor has an inner housing where a detection system is embedded, an outer housing in which the inner housing is accommodated, an outer-housing fixing arrangement, and a spring. One end of the outer housing is opened to the side of the windshield, and is engaged with the windshield through the outer-housing fixing arrangement. The inner housing is held in the outer housing in such a state that the inner housing is movable with respect to the windshield.
The spring is arranged between the outer housing and the inner housing, to press the inner housing toward the windshield. That is, in the conventional rain sensor, an inner-housing pressing arrangement (called spring-laminating type) where the inner housing and the spring are sequentially arranged between the windshield and the outer housing. In this case, because the spring is arranged between the outer housing and the inner housing, the height of the rain sensor is increased. Thus, the size of the rain sensor is increased.
With reference to JP-A-2001-141645 which has the same applicant as the present application, a raindrop detection device (rain sensor) is provided to restrict the above-described disadvantages of the conventional rain sensor.
According to JP-A-2001-141645, the raindrop detection device has an outer housing, an inner housing, a fixing member, a fixing unit, and a spring. In this case, a pair of springs which are bended are respectively arranged at the two ends of the inner housing. The middle portion of the outer housing protrudes to the front side to be used as a guide. The spring is pressed by the guide to the side of the windshield, so that the inner housing is pressed against the windshield. That is, a construction of a spring-siding type is used.
In this case, the springs are respectively arranged at the two ends of the inner housing. That is, springs are respectively arranged at two sides of a signal detection system (specially, light path system) of the inner housing. Therefore, the spring accommodating space and the light path system of the inner housing can be overlapped to the surface direction of the windshield. Thus, the height (of front-rear direction) of the rain sensor can be reduced.
However, in the above-described rain sensor of the spring-siding type, the variation of the detection signal due to a temperature change or a change over time is large, though the rain sensor becomes compact. In this case, the two ends (of left-right direction) of the outer housing are supported by the windshield through the fixing unit, and the middle portion (of left-right direction) of the outer housing presses the spring toward the windshield. The pressing force varies due to the temperate change and the change over time of the mechanical property and the shape of the outer housing. As a result, there occurs a variation in the shape of the light path portion and the like so that the detection level is changed. Thus, the detection accuracy is deteriorated.